It is planned to further study the conditions which are responsible for or can alter in vitro transformation with specific emphasis on the use of UV, X-rays, combinations of carcinogens of different classes, the combination of in vivo - in vitro systems, and the effect of stimulators and inhibitors of mixed function oxidases on toxicity and transformation. The mechanisms of early events of carcinogen-cell interaction will be studied. Investigation of different proximate carcinogens and metabolic inhibitors is planned. In addition, special emphasis will be given to the requirements necessary to obtain neoplastic transformation of human fetal cells. The testing for the tumorigenicity of cells exposed to different agents for different time intervals will continue.